Membrane voltage (Vm) and ion currents of rat mesangial cells in primary culture were measured with the patch-clamp technique in the fast whole-cell configuration. Vm was -44 +/- 1 mV (n = 138). A reduction of the osmolality from 290 to 190 mosmol/kg depolarized Vm from -44 +/- 1 to -29 +/- 1 mV (n = 118) and increased the inward and outward conductances (Gm) from 14 +/- 2 to 39 +/- 4 nS and 13 +/- 2 to 37 +/- 4 nS (n = 84), respectively. During the hypotonicity-induced depolarization the cell capacitance increased significantly from 33 +/- 3 to 42 +/- 4 pF (n = 40). The effect of hypotonic cell swelling on Vm was increased in a bath with a reduced extracellular Cl- of 32 mmol/l (by 71 +/- 4%, n = 23), indicating that a Cl- conductance was activated. The permselectivity of this conductance was I- > or = Br- > Cl-. The Vm response was not affected in the presence of a reduced extracellular Na+ of 5 mmol/l (n = 13) and was inhibited in a solution with reduced extracellular Ca2+ concentration (by 63 +/- 9%, n = 14). In microfluorescence measurements with the Ca2+-sensitive dye fura-2 hypotonic cell swelling induced a sustained increase of the intracellular Ca2+ activity, [Ca2+]i (n = 19). The increase of [Ca2+]i was completely inhibited when the extracellular solution was free of Ca2+. The Vm response to hypotonic cell swelling was not attenuated in the presence of the L-type Ca2+ channel blockers nicardipine (n = 5), nifedipine (n = 5) and verapamil (n = 5) (all at 1 micromol/l). The data indicate that in rat mesangial cells, osmotic swelling induces a Ca2+ influx from extracellular space. This Ca2+ influx activates a Cl- conductance resulting in a depolarization of Vm. The enhanced Cl- conductance may lead to KCl extrusion and hence regulatory volume decrease.